In 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE (hereinafter, referred to as LTE)), Orthogonal Frequency Division Multiple Access (OFDMA) is adopted as a downlink communication scheme, and Single Carrier Frequency Division Multiple Access (SC-FDMA) is adopted as an uplink communication scheme (e.g., see NPL-1, NPL-2, and NPL-3).
In LTE, a base station apparatus for radio communications (hereinafter, abbreviated as “base station”) performs communications by allocating a resource block (RB) in a system band to a terminal apparatus for radio communications (hereinafter, abbreviated as “terminal”) for every time unit called “subframe.” The base station also transmits downlink control information (i.e., L1/L2 control information) for the notification of the result of resource allocation for downlink data and uplink data to the terminal. The downlink control information is transmitted to the terminal through a downlink control channel such as a Physical Downlink Control Channel (PDCCH).
Here, the base station controls, on a subframe basis, the amount of resource (that is, the number of OFDM symbols used as a PDCCH region) of a resource region used for transmission of the PDCCH (hereinafter, referred to as “PDCCH region” in some cases), in accordance with the number of allocation target terminals and the like. This control is performed by notifying a control format indicator (CFI) transmitted using a Physical Control Format Indicator Channel (PCFICH), from the base station to the terminal. The CFI indicates the number of OFDM symbols used as the PDCCH region, starting from a leading OFDM symbol in a subframe. That is, the CFI represents the scale of the PDCCH region. The terminal receives the PCFICH, and receives a PDCCH according to the detected CFI value.
Each PDCCH also occupies a resource composed of one or more consecutive control channel elements (CCEs). In LTE, the number of CCEs occupied by a PDCCH (the number of concatenated CCEs: CCE aggregation level) is selected from 1, 2, 4, and 8 depending on the number of bits of downlink control information or the condition of a propagation path of a terminal. In LTE a frequency band having a system bandwidth of up to 20 MHz is supported.
Allocation control information transmitted from a base station is referred to as downlink control information (DCI). If a base station allocates a plurality of terminals to one subframe, the base station transmits a plurality of items of DCI at a time. In this case, in order to identify a terminal to which each item of DCI is transmitted, the base station transmits the DCI with CRC bits included therein, the bits being masked (or scrambled) with a terminal ID of the transmission destination terminal. Then, the terminal performs demasking (or descrambling) on the CRC bits of a plurality of items of possible DCI directed to its own terminal with its own ID, thereby blind-decoding a PDCCH to detect the DCI directed to its own terminal.
DCI also includes resource information allocated to a terminal by a base station (resource allocation information) and a modulation and channel coding scheme (MCS). Furthermore, DCI has a plurality of formats for uplink, downlink Multiple Input Multiple Output (MIMO) transmission, and downlink non-consecutive band allocation. A terminal needs to receive both downlink allocation control information (i.e., allocation control information about a downlink) which have a plurality of formats and uplink allocation control information (i.e., allocation control information about an uplink) which have one format.
For example, for the downlink allocation control information, formats of a plurality of sizes are defined depending on a method for controlling a transmission antenna of a base station and a method for allocating a resource. Among the formats, a downlink allocation control information format for band allocation in which RBs with consecutive numbers are allocated (hereinafter, referred to as “consecutive band allocation”) (hereinafter, simply referred to as “downlink allocation control information”) and an uplink allocation control information format for consecutive band allocation (hereinafter, simply referred to as “uplink allocation control information”) have the same size. These formats (i.e., DCI formats) include type information (for example, a one-bit flag) indicating the type of allocation control information (downlink allocation control information or uplink allocation control information). Thus, even if DCI indicating downlink allocation control information and DCI indicating uplink allocation control information have the same size, a terminal can determine whether specific DCI indicates downlink allocation control information or uplink allocation control information by checking type information included in allocation control information.
The DCI format in which uplink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 0” (hereinafter, referred to as “DCI 0”), and the DCI format in which downlink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 1A” (hereinafter, referred to as “DCI 1A”). Since DCI 0 and DCI 1A are of the same size and distinguishable from each other by referring to type information as described above, hereinafter, DCI 0 and DCI 1A will be collectively referred to as DCI 0/1A.
In addition to the DCI formats described above, there are other formats for downlink, such as: a DCI format 1 (hereinafter, referred to as DCI 1) for band allocation in which RBs with non-consecutive numbers are allocated (hereinafter, referred to as “non-consecutive band allocation”); DCI formats 2 and 2A for allocating spatial multiplexing MIMO transmission (hereinafter, referred to as DCI 2, 2A); a format of downlink allocation control information for allocating beamforming transmission (“beamforming allocation downlink format”: a DCI format 1B); and a format of downlink allocation control information for allocating multiuser MIMO transmission (“multiuser MIMO allocation downlink format”: a DCI format 1D). DCI 1, DCI 2, DCI 2A, DCI 1B and DCI 1D are formats that are dependent on a downlink transmission mode of a terminal (for non-consecutive band allocation, spatial multiplexing MIMO transmission, beamforming transmission, multiuser MIMO transmission) and configured for each terminal. In contrast, DCI 0/1A is a format that is independent of a transmission mode and can be used for a terminal having any transmission mode, i.e., a format commonly used for every terminal. If DCI 0/1A is used, single-antenna transmission or a transmission diversity scheme is used as a default transmission mode. Meanwhile, the following formats are studied for uplink allocation: a DCI format 0A for non-consecutive band allocation; and a DCI format 0B for allocating spatial multiplexing MIMO transmission. These formats are both configured for each terminal.
Also, for the purpose of reducing the number of blind decoding operations to reduce a circuit scale of a terminal, a method for limiting CCEs targeted for blind decoding for each terminal has been under study. This method limits a CCE region that may be targeted for blind decoding by each terminal (hereinafter, referred to as “search space”). As used herein, a CCE region unit allocated to each terminal (i.e., corresponding to a unit for blind decoding) is referred to as “downlink control information allocation region candidate (i.e., DCI allocation region candidate)” or “unit region candidate targeted for decoding.”
In LTE, a search space is configured for each terminal at random. The number of CCEs that forms a search space is defined based on the number of concatenated CCEs of a PDCCH. For example, the number of CCEs forming search spaces is 6, 12, 8, and 16 in association with the number of concatenated CCEs of PDCCHs 1, 2, 4, and 8, respectively. In this case, the number of unit region candidates targeted for decoding is 6 (=6/1), 6 (=12/2), 2 (=8/4), and 2 (=16/8) in association with the number of concatenated CCEs of the PDCCHs, 1, 2, 4, and 8, respectively. In other words, the total number of unit region candidates targeted for decoding is limited to 16. Thus, since each terminal may perform blind-decoding only on a group of unit region candidates targeted for decoding in a search space allocated to its own terminal, the number of blind decoding operations can be reduced. A search space in each terminal is configured using a terminal ID of each terminal and a hash function for randomization. A terminal-specific CCE region is referred to as “UE specific search space (UE-SS)”.
The PDCCH also includes control information for data allocation, the information being common to a plurality of terminals and notified to the plurality of terminals at a time (for example, allocation information about downlink notification signals and allocation information about signals for paging) (hereinafter, referred to as “control information for a common channel”). To transmit the control information for a common channel, a CCE region common to all the terminals that are to receive downlink notification signals (hereinafter, referred to as “common search space: C-SS”) is used for the PDCCH. A C-SS includes six unit region candidates targeted for decoding in total, namely, 4 (=16/4) and 2 (=16/8) candidates with respect to the number of concatenated CCEs, 4 and 8, respectively.
In a UE-SS, the terminal performs blind-decoding for the DCI formats of two sizes, i.e., the DCI format (DCI 0/1A) common to all the terminals and the DCI format (one of DCI 1, DCI 2 and DCI 2A) dependent on a transmission mode. For example, in a UE-SS, the terminal performs 16 blind-decoding operations in each of the DCI formats of the two sizes as described above. A transmission mode notified by the base station determines for which two sizes of the DCI formats the blind decoding is performed. In contrast, in a C-SS, the terminal performs six blind-decoding operations on each DCI format 1C, which is a format for common channel allocation (hereinafter, referred to as “DCI 1C”) and DCI 1A, (i.e., 12 blind decoding operations in total) regardless of a notified transmission mode. Accordingly, the terminal performs 44 blind decoding operations in total for each subframe.
DCI 1A is used for common channel allocation and DCI 0/1A used for terminal-specific data allocation have the same size, and terminal IDs are used to distinguish between DCI 1A and DCI 0/1A. Thus, the base station can transmit DCI 0/1A used for terminal-specific data allocation also in a C-SS without an increase in the number of blind decoding operations to be performed by the terminals.
Also, the standardization of 3GPP LTE-Advanced (hereinafter, referred to as LTE-A), which provides a data transfer rate higher than that of LTE, has been started. In LTE-A, in order to achieve a downlink transfer rate up to 1 Gbps and an uplink transfer rate up to 500 Mbps, a base station and a terminal capable of communicating at a wideband frequency of 40 MHz or higher (hereinafter, referred to as LTE-A terminal) will be introduced. An LTE-A system is also required to support a terminal designed for an LTE system (hereinafter, referred to as LTE terminal) in the system in addition to an LTE-A terminal.
Additionally, in LTE-A, to achieve an increased coverage, the introduction of radio communication relay apparatus (hereinafter, referred to as “relay station” or “Relay Node” (RN)) has been specified (see FIG. 1). Accordingly, the standardization of downlink control channels from base stations to relay stations (hereinafter, referred to as “R-PDCCH”) is under way (e.g., see NPL-4, NPL-5, NPL-6, and NPL-7). At present, the following matters are being studied in relation to the R-PDCCH. FIG. 2 illustrates an example of an R-PDCCH region.    (1) A mapping start position in the direction of time-axis of an R-PDCCH is fixed to a fourth OFDM symbol from a leading symbol of one subframe, and thus does not depend on the rate at which a PDCCH occupies OFDM symbols in the time-axis.    (2) Each R-PDCCH occupies a resource formed by one or more consecutive relay-control channel elements (R-CCEs). The number of REs that form one R-CCE is different on a slot basis or on a reference signal arrangement basis. Specifically, at Slot 0, the R-CCE is defined as a resource region (excluding regions to which reference signals are mapped) having a range from the third OFDM symbol to the end of Slot 0 in the time direction and having a range of a one-RB width in the frequency direction. Further, at Slot 1, the R-CCE is defined as a resource region (excluding regions to which reference signals are mapped) having a range from the beginning of Slot 1 to the end of Slot 1 in the time direction and having a range of a one-RB width in the frequency direction. Note that, for Slot 1, it is also proposed to divide the above-mentioned resource region into two and define each divided region as one R-CCE.